The growth of solar cells with self-assembled quantum dot layers typically requires that multiple layers of self-assembled quantum dots, intercalated (interposed) with barrier material layers, be formed atop a substrate. As the material used in forming self-assembled quantum dots layer has a different lattice constant than the material used in forming the barrier material, strain is present in the multiple layers of self-assembled quantum dots, and in the intercalated (interposed) barrier material layers. As such, the number of self-assembled quantum dot layer/barrier layer units that can be formed atop each before the onset of lattice defects, such as, for example, dislocations, can be limited.
The limit on the number of quantum dot and barrier layers can be made large by choosing appropriately the thickness and composition of the quantum dot and barrier layers by, for example, ensuring that the average lattice constant of the region (volume) containing the quantum dot layers and the barrier layers remains substantially the same as that of the substrate. The region containing the quantum dot layers and the barrier layers can be referred to as the quantum material region or as the region containing the quantum dots or the self-assembled quantum dots.
As used herein, the expression “average lattice constant” means the average of the nominal lattice constants of the materials in the solar cell, weighed in accordance with (as a function of) the amount (thickness, number of bonds) of each material in the solar cell. When the average lattice constant of the region containing the self-assembled quantum dots remains substantially the same as that of the substrate, the in-plane lattice constant remains the same throughout the quantum material region, i.e., the quantum material region is coherently strained, and the average of the vertical lattice constant will be substantially equal to the substrate lattice constant. This means that the quantum material is coherent with the substrate, and minimal strain relaxation is present in the form of defects such as, e.g., dislocations, anti-sites, substitutional defects, vacancy defects, or point defects.
The expression “in-plane lattice constant” refers to the plane perpendicular to the growth direction of the quantum dot and barrier layers. The expression “vertical lattice constant” refers to the lattice constant in the growth direction of quantum dot and barrier layers. Unless the average in-plane lattice constant is substantially equal to that to the substrate, strain can accumulate and defects can eventually form in the quantum material region to relax the strain. Such defects can reduce the minority carrier lifetime, which can be detrimental to device (e.g., a solar cell) performance.
Nevertheless, even when care is taken to ensure that the average in-plane lattice constant is substantially the same as that of the substrate, the quality of the quantum material in the case of a stack with multiple repeats of intercalated self-assembled quantum dot layers and barrier layers can be compromised by such unwanted defects if no particular precaution is taken in the layer sequence.
Additionally, the stack of intercalated self-assembled quantum dot layers and barrier layers can be sensitive to process condition variations during their epitaxial growth, leading to unwanted defects (e.g., dislocations). The lattice defects generally lead to a decrease in performance metrics such as, for example, conversion efficiency, fill factor, and open-circuit voltage, and/or the defects can reduce the number of devices per manufactured wafer, that meet pre-determined performance criteria per (i.e., the defects can lead to poor manufacturing yield).
Improvements in solar cells having self-assembled quantum dot layers is therefore desirable.